Two wire current transmitter responsive to a resistance sensor input signal

ABSTRACT

Circuitry including a resistance sensor is excited by an external power source. A reference voltage is established, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the circuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a non-linear function by proper selection of certain resistors.

United States Patent [191 Grindheim 111 3,859,594 1 1 Jan. 7, 11975 TWO WIRE CURRENT TRANSMITTER RESPONSIVE TO A RESISTANCE SENSOR INPUT SIGNAL [75] Inventor: Earl A. Grindheim, Minneapolis,

Minn.

[73] Assignee: Rosemount Inc., Minneapolis, Minn.

[22] Filed: Feb. 24, 1970 [21] App]. No.: 14,748

Related US. Application Data [63] Continuation of Ser. No. 661,988, Aug. 21, 1967,

abandoned.

[52] US. Cl. 324/57 R, 73/88.5 R, 73/362 AR, 323/75 N, 324/62, 324/65 R, 324/DlG. 1, 340/186 [51] Int. Cl G01! 27/00 [58] Field of Search 73/362 AR, 88.5 R, 362 R; 323/75 H, 75 N,9; 340/186; 324/57, 62,

65 R,99,4,22,61, 123,DIG. l; 330/9,

[56] References Cited UNITED STATES PATENTS 2,930,224 3/1960 Bodner et a1. 324/65 X 2,974,279 3/1961 Barry et a1. 324/62 3,177,427 4/1965 Kuntz et a1. 324/61 3,286,171 11/1966 Fluegel et a1.

3,302,106 1/1967 Shaw 324/99 X 3,322,971 5/1967 Chung-Chuan Liu 307/297 3,408,584 10/1968 Miller, Jr. et a1. 330/9 3,483,476 12/1969 Kobayashi et a1. 330/10 3,517,556 6/1970 Barker 73/362 AR Primary ExaminerStanley T. Krawczewicz Attorney, Agent, or FirmDugger, Johnson & Westman [57] ABSTRACT Circuitry including a resistance sensor is excited by an external power source. A reference voltage is estab' lished, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the cir cuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a nonlinear function by proper selection of certain resistors.

23 Claims, 3 Drawing Figures lNPUT CURRENT DIFFERENTIAL D-C-SOURCE CONTROLLER PATENTEU M 1 I975 SHEET 10F 2 womDOw 6 0 IN VENTOR. 191?! 9. 'E/A/O/VE/M KJM K (mm,

PATENTEU JAN 7 I975 SHEET 2 OF 2 INVENTOR. 541?; 4. Gel/v0 l/E/M lazy/r Total current drawn by the network 10 is designated I, and is shown as being received by line 15 which connects the output of a differential input current controller 16 to terminal 14. The total current I, leaves network l by line 17 which connects junction 18 between R, and R to power return terminal 19. The total current I, is supplied to the current controller 16 from input terminal 20 which is connected to 16 by line 21. The signal terminals 11 and 13 connect to current controller 16 by lines 22 and 23 respectively. Controller 16 responds to a voltage difference between lines 22 and 23 in such a manner to reduce any such voltage to substantially zero by adjustment of total current I, and there is substantially zero current drawn by lines 22 and 23. A direct current source 24 and a load 25 are shown serially connected across terminals 19 and 20.

In operation a change in one or more of the resistors R, through R as a result of a measurand change (measurand refers to the quantity of physical variable being measured such as temperature or strain) gives rise to a voltage signal between lines 22 and 23 which causes the controller to adjjust the current level lto reduce the voltage signal to zero. The change in current I, is thus a measure of change in resistance of one or more of the resistors -R, through R, and this change in current is monitored by load 25 which may be a recorder for example. For an understanding of the circuit response assume that R, is zero. In this case the voltage across terminal 14 and junction 12 is a constant value V regardless of the current flow through diode V,. Accordingly the voltage on line 23 is held constant by V, so long as the ratio between R and R does not change. Then assuming R, is constant and R is allowed to vary, the voltage on line 22 will rise with an increase in R.,. In order to have a self balancing response this increase in voltage on line 22 must give rise to an increase in current I, from controller 16. The increase in current will not affect the voltage on line 23 (since it is assumed that for this example R is zero) however it will have an opposing effect to the voltage on line 22 since it will give a change in voltage across resistor R of opposite sign to the change in voltage across R due to the assumed increase in'resistance of R Then to consider the effects ofa finite value of R to be of value such-that the voltage drop across R is small compared to V,. Then an increased current through the series combination of V, and R caused by an increase in R, for example, raises the voltage on line 23 by an amount proportional to the increase in voltage across R The same percent increase would also be felt as a part of the voltage change on line 22. Since the voltage on line 22 is assumed to be greater than that on line 23 due to an increase in R, the effect of the increased voltage across R, is to require still more current I, to flow to reduce the voltage difference to zero between lines 22 and 23. Since R introduces a correction or adjustment based upon the change in current I, its effect is a higher order effect and it may be used as such to selectively shape the relation between current I, and resistor R, for example. Analysis of the network gives the following expression between I, and the various resistors shown: 2 (R4 5) 1 3]/( l 5( 2 3) R6(R3 5 z 4 s) (I) It may be noted that if R, is zero the relation between I, and R, is linear and further that if R, is not zero the current I, will respond to positive changes in R in an increasingly senssitive manner. It may also be noted that an increasing R would result in l, increasing at a less than linear rate if R, is zero and at a rate selectible from less than linear to greater than linear if R is finite. An increasing R, or R would give a decreasing current I, and the rate ofdecrease would reducewhether or not R was zero however the magnitude of reduction depends somewhat-on R v As a further example consider linearization of the relation between I, and temperature when a platinum'resistance thermometer is used as the measurand sensing instrument. Picking R, as the thermometer the relation between resistance and temperature is R =R,,(1+aT-BT (2) for tempertures zero degrees Celsius and higher, where R, is the resistance at 0 C, T is temperature in degrees Celsius and a and Bare constants. Substituting expres- -sion(2) into expression (1 expanding terms and solving for the condition causing disappearance of terms involving T in the numerator gives i --2- 1 R, R, a I (3) Accordingly it is only necessary to satisfy expression (3) to achieve a linear relation between total current I, and temperature when a resistance thermometer such as platinum is used as R.,. Similar analysis may be made for the case where R, is a platinum thermometer. If the rate of resistance change as a function of temperature increases, as it does for a nickel wire thermometer for example, the element may be shunted by a constant resistance to linearize the response and it may then be used in place of R, with a zero value for R,,. A thermistor or thermistor network involving a series-shunt combination of resistors having. a negative change of resistance with temperature would preferably be used in place of R, or R In some cases it is desirable to have more than one resistor respond to the measurand. Resistance strain gage measurements commonly employ at least two resistors, one increasing with strain and one decreasing, and these would be preferably located in adjacent positions of network 10. If a temperature difference as sensed by two thermometers having like characteristics is to be measured these resistance thermometers would also be located in adjacent portions of network 10, for example in place of R and R A range or span adjustment can be conveniently made by adjustment of magnitude of R and zero may be adjusted by R,, for example.

In some instances it is desirable to use the circuitry shown for converting a low level voltage signal such as a thermocouple output to a controlled current signal. A thermocouple or other voltage signal may be introduced in series with one of the resistors R, through R or by connection in series with, or across, current controller signal lines 22 and 23. Resistor R may be selected to be temperature responsive also so that it serves as a reference junction compensation for a thermocouple having its reference junction adjacent R.,. In all such cases the differential input current controller responds to a voltage signal across lines22 and 23 in such a manner to reduce that voltage to zero and resulting current drawn by the circuit and available for measurement across load 25 bears a predetermined relation to the voltage signal and its origin.

a'ge levels-throughoutthe?circuit;While a straightline 83 which is'connected to the output of multivibra- .tor 70 A positive signal on line 83 cuts off transistor 43 '1, erence 'voltage'v cis fshown inPFlG. The system of and turns on transistor 42 thereby effectively referenc- FIG. 2 is a carrier amplifier type-controller giving a I ing the output of amplifier-50 to power line 81. The al- "high degree offreedo'min choice'of direct current Volt 5 ternate'negative signal on line 83 cuts off 42 and turns on"43 thereby coupling the output of amplifier 50 to the outputterminal 45 of demodulator 40 A detailed schematic of a r differential input controller together with network resistors Rah-rough R and ref- DC 'ornon-carrier type controller Ii y be usedthe car-"f feet.transistor:42;connected" in-series.withIR field effect. transi storj4 3 at junction 44 .The' ource of -rier amplifier'jtype controller generally gives overall 7 high ei" performar'icethan would beavailable with-a di-' rect cur rent coupled' amplifierl Q The networki resistors 'R through and reference voltage V; are connected in F1612 in the same arrangement of FIG l howev'er the main suppty of controller,

feedback current-is n'ovv' delivered to the network .at terminal 26 between Rgand the anode-of Zener diode V The circuit is arranged to' receive direct current "power at terminal which connects'to a current controller stage designatedgenerally at 30. The current is controlled by stage 30 inresponse-to a signal from a demodulator which in tu'rn'connects to the output of a differential arriplifier'SO. Amplifier SQresponds to the signals across network terminals-ll and 13 which are H coupledto amplifier by a modulator 60. Modulator multivibrator 70 which i's asquare wave, symmetrical,

Amplifier 50 is a direct current integrated circuit differential amplifier having its output capacitively coupled-to resistor 53 and thence to junction 44. Input power is obtained from line 81 and power return is to line 82. Amplifier signal input terminals 51 and 52 are capacitively coupled to modulator output terminals 61 and 62 respectively. The series combination of resistor 54, capacitor and resistor 56 is connected between input terminals 51 and 52. The output of amplifier 50 is D.C. connected to the junction between resistor 54 and capacitor 55 by means of degenerative feedback resistor 57. This degenerative feedback of direct current signals insures that unwanted error signals such as thermoelectric"potentials at the input terminals will have little effectlon-theamplifier control signal. An al- I ternating-voltage' signal-from modulator will be am- 60 and demodulator-'40 areisynchronously driven by a;

' free-running type multivibrator;Themodu'lator 60-and demodulator 40fmay be referred'toas choppersand the multivibrator isa specific example of a chopper'driver generating a wave form commonly referred tofas chopperdrive. The total current drawn bythe circuit is'ef The amplifier 5.0 is-operated from'ajbalanced voltage supplied by line s;8 l'j an'd'82 which is maintained substantially constant bys's'e'ries connectedZener diodes 84 fective in'obta'ining a zero voltagejdifference' across network terminalsll and 13 in-the samemanner, as described with reference to'F IG. 1 and conseque ntly the current I, which would bemeasured by'a serially con- Q nected loa'd as-iwas shown: in FIG. I isj accurat-ely ,de-

scribed; by expression (1-) when'the circuit is operating i in a balanced-iconditio'nf'x Current co'ntrol'lerfit) includes a pair.of ti'ansistors3l :fand BZ -having their collectors}; connected to. input v v powerte'rmihal-lfli These transistors for-m aDarlington I amplifier} sinceftheemitter of32f c'onnec ts through ree sistjo'rf3 3to'powerline-81 which'line is the-mainsource for the other c ircu'it' elements. Controlle'ff-SO :"alsoin: v eludestransistorv 34jand Zener diodej35which'operate' r vto give a substantiallyconstant curren'tiinthj'e' base to jemitter circuits of: '31 and 321 venthoughthej input -powersuppliedtojterminal 20fmay vary considerably" and 85. These dii'odesjare'jofithe same type and same voltage breakdownjTh junction between the cathode of85 'and the anode ofl84 isconnected tothe junction between capacitor 55 zanjdresistor'5 6- at the input to qa mplifier 50 byline88therebymaintainingthe-input ofamplifier 5 0 rnidway b'etvi/ee n' the voltage on lines 81 andf82;

e [Modulator I 60fincludesflfield greatransistor 63 which sisalternately made conducting and 'nonconducting byi-the multivib'rator output capacitively coupled t -theQga'te of 'tra'risistor 63 from line 86. The output-connectionsof transistor 6 3 connect directly to '.modulator output 't r- 'min'als162fand 6l r'espectively;

:across-amplifier terminals Theseoutput terminalsare,resistance coupled to network output terminals ['1' and 1'3"re spectivelyjsoithat a outputsignal arising at terrriinals1 1," and l3 from anetwo r-k unbalanc; is alternately shorted and applied established by multivib rla'to'r 'l ovgi in voltage" level. Thefbas'e'. of transistor 31 connects to the collectorjo'f'34 and: connects through-resistor 36 Jto I T j lp -s 'bfiw "th :Caih f eil j'diQds 1' f ndresistor 3?Z'.fl The"other endofresisito'r 37'. corinects to inputterminal 2.0. The emitter'of transistor-$4 and j anode of diode'SS connect. to line8'1jand{tlie base of.

I transistorl'34 connects totheoutp ut" of demodulator 40 "throughresistor 41: ',A" .capacito'r{38 is connected] be tween the baseoftransistor 34 a n'd "the-'emitter'-of trann ents that may appejarqat'transis tor'3 4tj i sistor 32' in order-toshunt any high frequencylcompofl;

The demodulator 401 includes 'anNl-c annel field efchannel jtransistor 42 conn'ectsv to'line 81 and the soufcej-ioftran' I component drawn by these elementsis active inthe sistor 43 connects to resistor-41' at demodulator outp'ut terminal 45. Resistors are respectivelyconnectedlfrom gate to source of transistors 42'and 43 and thetransis- Multivibrator 70 receivespowe'rlfrom line 81 andhas a power. return to line. 82 {pair of transistors oscillating iri' a-j'continuous manner I v and, as shown, is of conventionaled esign which requires;

The multivibra'to'r includes a .no elaboration;

The basic operation of the circuit ofFl G 2 is similar to the description given inreference'toFlGg l. Amplirfier SO and current controller function to maintain 1 zero voltage between network-terminals l landl3 and the total current drawn by thefcli'r' cuit-fis"related to the r network resistor-s R, through R andreferencevoltage i 1V by expression (1). Amplifier 50 and multivibrator 70Qare each powered between lines 81 and482 which are maintained at substantially constant voltage by Zener diodes 84 and 85. Consequently the current network balance since the current re'turn is from line 82 through linearizing resistor R and current feedback resistor R to output terminal-l9. This current compoj nent is typically small and relatively constant and the and 52 atthe frequency main signal current is developed by virtue of network unbalance resulting in a change in current through Zener diodes 84 and 85 and thence over line 82 through resistors R and R to output terminal 19. Resistor 87 couples power from line 81 to the cathode of reference source V which in turn supplies the network in the manner described with reference to FIG. 1. Resistor 87 substantially blocks the balancing current supplied from controller 30 from passing through reference element V, thereby minimizing any change in V, which might otherwise be caused by relatively large changes in current through V As an example of operability it was desired to deliver an output current varying from 10 to 50 milliamps for a temperature change from C to 100 C as measured by a platinum resistance thermometer. A sensor resistance of nominally 100 ohmsat 0 C was selected and was simulated by a manually variable resistor substituted for R Other network resistor values were 1.008 ohms for R 2.26 ohms for R,,, 90.78 ohms for R and 6187.2 ohms for each of R amd R Reference source V, was a IN-827 Zener diode having a nominal voltage of 6.2 volts and Zener diodes 84 and 85 were type IN- 4739 controlling at about 8.4 volts. Resistor 87 was 1850 ohms and the input voltage from the DC. source 24 was approximately 60 volts. Amplifier 50 was a type 709C direct current operational amplifier manufactured by Fairchild Semiconductor, Mountain View, California and the other components were of size and type to maintain the various circuit elements within their design ranges of operation.

When resistance R was varied to correspond to the well known resistance change of platinum with temperature the results of Table l were obtained.

The results in Table l are one example of the close agreement which is obtained between a measurand and output current for the circuits herein disclosed. Although the example was for a current range of -50 ma'for a 100 ohm temperature sensor and a 100 C range it is apparent that the circuits are suitable for operation over a wide range of variables and the current range achieved may also be selected over a wide range while using the circuits which are described and illustrated herein.

An alternate schematic of the general network 10 shown in FIG. 1 is shown in FIG. 3. The network of FIG. 3 is substantially equivalent to network 10 and the operation follows the same formula relating total current, reference source V and the resistors making up the network. The network of FIG. 3 was derived from 10 by transformation of the wye circuit comprising R R and R of FIG. 1 to the delta" circuit comprising resistors, R R and R of FIG. 3. Expression (1) may also be applied to the network arrangement of FIG. 3 where the following transformations apply:

7 s e s a s e)/ s s s a s s a a)/ s The network arrangement of FIG. 3 may be substituted directly into the circuit of FIG. 1 or the circuit of FIG. 2 by connecting terminals ll, l3, l4 and 19 to the terminals of like numbers in FIG. 1 or FIG. 2 anddisconnecting the corresponding networks shown in those Figures. The current derived from differential input current controller 16 may be applied directly to terminal 14 as shown in FIG. 1 but in the preferred embodiment the major portion of the controlled current isdelivered to the network at terminal 26 as was shown in the circuit description of FIG. 2.

I claim:

1. A measuring system comprising a pair of terminals, a source of direct current and a load, means to connect the source and load in series between said current terminals, means to provide a variable voltage signal which varies in magnitude as a function of a variable condition to be measured, a current adjusting controller means having a voltage responsive input coupled to said variable voltage means for receiving said variable voltage signal and adjusting the current through said terminals as a function of the magnitude of the variable voltage signal at said input, said controller means being connected across said terminals and energized solely from current supplied through said terminals, resistance means connected to said current terminals to provide a voltage feedback which is a function of total current flowing through said terminals, and means connecting said resistance means to said input of said controller means to provide the feedback voltage as determined by the current through said resistance means to tend to balance a change in the variable voltage signal at said input occurring due to a change in magnitude of said variable condition and to stabilize the adjusted current through said terminals at a new magnitude which is representative of a change in magnitude of said variable condition.

2. The measuring system of claim 1 wherein there is provided resistance network means for providing a variable voltage signal, said network means being connected to said terminals, energized solely through said terminals, and including a first resistor which varies in value as a function of the variable condition to be measured, and comprising at least portions of the first mentioned means to provide a variable voltage signal, the first mentioned resistance means and the means connecting the resistance means to said input.

3. The measuring system of claim 2 wherein said resistance network means includes additional resistance means, said means to connect said first mentioned resistance means to said input including means to connect said first resistor, said first mentioned resistance means and said additional resistance means in a bridge circuit and to provide bridge voltage output terminals that are connected to said voltage responsive imput.

4. The combination of claim 2 wherein the first mentioned resistance means comprises a second resistor and the resistance network means additionally comprises third, fourth, and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors.

5. The combination of claim 4 and a voltage reference means to provide a reference voltage for said network means connected between a junction intermedi- 9 ate said fourth and fifth resistors and a junction' inter mediate said second and-third resistors,

. 6. The combination of claim 2 whereinthe first me ritionediesistance means comprises a seCQnd resistor voltage reference-'means is connected for-current conduction from the junction er the third andsecond resistors to the junction of the fourth and firth resistors, the

v junction offth'eifirst and'fou'rth resistors comprising a v first'voltage terminala'nd the .junction of thefifth'and third resistors comprising a-second voltage terminal, and the junction of the first and second resistors being coupled to one of-saidfirst mentioned pair of terminals and the other of Said'first mentioned pair of terminals being connected tothef junction of said second and third resistors.

I, 7. The combination :of clairnffi and a voltage reeference' means connectedfrtofisaid resistance network portion that is dependent on the magnitudeof the variable voltage at said input portion, means coupled to the output portion and the second end portions of said wires for regulating the flow of current through the ,wires-as a function of the output voltage including feedback resistor means coupled across the second end portions of the wires for generating a feedack voltage de- 'j pendent upon the current flowing in said wires, and means for electrically connecting said feedback resistor means andto said first men tion'e'dpa'ir'of current termi-l.

, nals to provide a" reference voltage for said resistance network means in cluding avoitage' reference element connected to the 'junction'ofsaid fourth and fifth resistors'and a sixthresistorconnectedbetween theivoltage" reference element andth e'iunction of the third and the second resistors and wherein'the sixth resistor is,con-

'nected in series with the secondgresistor andthe-first.

mentioned pair of current terminals, 4 8. The combination'of claim' 7 wherein the voltage reference elementis a Zener diode."v Y 9.. The combination of claim;v wherein said resis},

tance network means includess'econdgthnd and fourth resistors connected in a. s mmetricall'brid arran' e l i v merit having one endfofsaid first resistor'cdnnected to U oitherof sald'wlres and through SXth reslstor tosaid secondvoltage terminal 1'2; Anje'lectrical current-transmitting system comprising a pjairfofcurrent terminals adapted for connecone end of said thirdjresis'torfand onee'nd oflsaid s'e'c." ond resistor being connected tojone e'ndof s'aid fourth 'means to ;the input portion of said amplifier means to tend to balance variations in magnitude of the variable vdlt age at 'said 'input portion.

11. The combination as specified in claim 10 wherein said means providing a voltage that varies in response l ftovariation of the parameter to be measured comprises a resistance,networkincluding a first resistor which varies in res-ist ance'as a function of a parameter to be measured,said feedback resistormeans comprising a second resistor in said resistance network, said resistance network further including third and fourth resisr tors, said first, second, third, and fourth resistors being connected in a symmetrical bridge arrangement having one end of saidfirst resistor connected to one end of saidthird resistor to provide azfirst voltage terminal, and one end ofsaid second resist r being connected to i one 'endof said fourth're'sistor to provide a second voltw age; te'rminaln and wherein the other ends of said first andbsec'ond resistorsa re;connected to one of said wires the other ends of said third and fourth resistors are connected together tolth'e other of said wires, said feedbacklrsistor means further including fifth and sixth re- .si'stors and a voltage reference'means for providing a reference voltagejfo'r said resistance network including a voltage reference element having onee end coninected ,to' th e -junction of said third and fourth resistors resistor; the other endsof saidffirst'aiid, second resistors being connected toi'one ofsaidtermina having one end connected. to theothe endof. said terminals, and a sixth resistor having-,one' end, connected to the junction of said secondand'fourth resistors and voltage referencemeans for providingr'a reference volta fifth resistor and the other end'c'on'necfed through said fifth resistor tion to a load; and 'power source that are in series; a resist'ance network, meanslconnected in series with said current terminals'to .b'epowered and excited solely therethrough for providing a first variable voltage signal' whichflvaries asa function of the magnitude of a j physica'l condition to be. measured; a feedback resisage reference element having one 'end jc'on'nected to the junction of said third and fourth'resi'storsiand the other.

--end connected to the other endiofslaidffifthresistbrlj and the other end of .said voltage-referenceelement} being connected to the other end of said;s ixth resistor.j IOLA transmitter for measuring a-variable voltage de-l pendent upon the value of a variableg'paranieter',first and second wires each havinga first end-portion a'nd'a second end portion, a transmitter connected across 'said second end portions, a measuring device,- current source means for providing the sole energyfsupply' to: the transmitter and the current-thatserves m transmita signal from the transmitter, saidfrneas'urin'g device:

and current source means being' connec 'ted in series across the first end portions of said wiresfsaid transmitter including means providing a voltage-that varies in response to the variation of the parameter tobe'mea sured, amplifier means having an input portion coupled I to said variable voltage providing means'and an output portion for providing an output voltage at saidoutput *tancejrneansjconnected to said resistance network means;for.p17oviding 'asecondvoltage signal which is a function of 'the' current.tih'rough 'said feedback resis- J- tancel meansynieans tobelpoweredtsolely through said terminals con nectedito the feedback resistance means anditoftheicurrent terminals to establish a current flow gpa tli' betweensaid terminals'arid'to provide a known 'portion'vof currentflowingf-be'tween said terminals through said feedback;resista'ncemeans including an "amplifier means havinga voltagefinput portion and an "toutput porti'on providing an output signal which isa function 'offthe voltage'at said voltage input portion, (and currenfcontrolandadjustment meanscoupled to said output portion to receive the output signal and to -adjust the current-in said-current flow path between said'terminals as afunction of said output signal; and

means to couple said resistance network means to said voltage input portion toprovide a voltage at said input portion of said amplifier-means which is a function of 7 said first and second voltage signals.

" fier means comprises a chopper driver havinga'drive'rv f output terminalgsaid means to couple-comprising a modulator means" having first and second input. t ermi-.

13. The combination 'ofclaim 'IZfurther character--- ized in that a voltage reference means is con'nected to said resistancenetwork means-inserijes with said feed vide a reference voltage for said resistance network means. H

14. The combination of claim 13 wherein said net work means includes a first resistor, and-saidvoltagej I reference means includes a fixed resistariceand a semi;

conductor reference element in series with saidfiXe'dQIO stagejhaving'a'first input terminal connected to the deresistance, the series combination of'said reference ele-IQ ment and fixed resistance being connected acrosssaid first resistor so that an increase in excitationvoltag'e creased current through said voltage reference means, 15. The combination of claim 12 whereinisaid ampli" 17. The transmitter of claim 16 wherein the current controller means comprises a chopper driver having a I v. "driver-output, a modulator having input terminals con- I back resistance means between said terminals topro nected'to the input circuit means, means coupling the 5 I fdriver o'utputto said modulator, an amplifier having an input and an ofutput and having its input connected to the modulator output, a demodulator having an input connected to the amplifier output, means coupling the driver output to the demodulator, and acurrent control "-rnod ul'ator'output, asecond input terminal, connected f to one of'said wires'and an output terminal connected toflthe otherof said jwires. across said first resistor is provided as aresult of an in --'I'l 8 The signal transmitting system of claim 16 whereinsaid'input circuit meanscomprises a resistance network means to'provide a variable voltage including v a-firstresistorwhich-changesinvalue in response to a '-,chang;i n' a variable'condition andsaid feedback resistorfrneansfcomprisesj a second resistor also coupled to 5 has connected to said resistance network means'to 2O is aidjresistor network means 'said resistance network load, current supply means for providi'nga'l -through said loadand wires connected, the load across first ends of said wires;.a;.crent trans thereof, .th'e'current-flowingthrough said wires'se as-the solepower supply to th'e'f-tran'smitte'r'a I serving atjtheiload to represent thesignal transmitte H j'from I said transmitter, 'said transmitter including;

input circuit meanscon'nected to saidtwo wires to pro vide a variable voltage which; tendsito change injre pouse to changes. in a variable condition constituting. -'th e 'info'rmationiftobe transmitted and voltage respon-' sive current controllerimeans. connected to. the two wires and the'in'p'ut circuit-means for adjusting th'ecur-f rent through said two wires inresponse to the voltage]; 1 supplied" by-t-he input circuit mea ns, feedback resistor meaQns'Cohnecte'd to said currentcon't'roller means and in series with' 'saidwir'es to have at least a known portion of load currentca'rriedby said wires flowing theresensevoltages in said network means, a third input ter- *minal capacitively connected to the drive output termin'al an'dIa modulator"outputfterminaltjan operational amplifier having an ifnput terminal connected to the modulator output-terminal and having an output termil nal; a dein odulatorjhaving a first" inputterminal connected to the amplifi e'rzoiitput terminalg-a second input terminal capacitive'lv;connected to the'fdriver output terminal and: having anoutput-terminal;{and .said cur: rent control and] adjustment meanslcomprisinga;vcur- 30 rent control stage havin g'fa fir'st fin'putl' terminal conn'ected to the-demodulatoroutputfterminal,a second current terminal connectedto'one'ofj thertransmitting system-pair of current'te r min-als and attliirdw-terminal gmea'ns" connected tojjpass current 'through portions"-of*5 ,said 'resistance network including said-*feedback;re sis tance means to the otherof thetransmitting system pair of currentterminals.

16. A signal transmitter comprising a pair. f wi'res; a dad current wit rnitter connected across saidjwires at th other-end through tojlprovide a feedback voltage: dependent on the magnitude of said load current, and means connectingsaid feedback resistor means'to theinput circuit means to provide a feedback voltage in opposition to the variablevolta ge so that the input voltage to the current controller means is dependentup on thetvariable voltage and the feedback voltage whereby adjusted load current flowing through said wires and load is representative of the magnitude of said variable co'n'ditioritv meansfurther including-third, fourth andfifth resistors,

all of s'aid resiistors'beingzconnected in a closed series nectedftbet ween.thesecond yand fifthiresi stors', and a "-voltageteferenceelement and a sixth resist orcon nected. in seriesb'e'tweenth'e junction ofsaidlfo urthrandi v fifth resistors. andith'e j unc'tiongofjs'ald second andthird' resistors, said means to coupleisai dfinput c'ircuit to said' coupling 'on'e of said wires and-n fthf 'r'e'sist o rs to carry? "d resistance net-work and means also coupling th'e unctionzo'f the volta'ge-reff two wires comprisingmeans to the junction of theifourth a first portion of the cur'ren erence element and the siXth1 resistor: to? said-one wire. to'xcarry a secondfportion of the t i rrent-inthe one wire to the resistance network sa id -l a gbei ng, {connectedthrough-portions. of sa'id "tance" fne-twiorkdo thejother wi'reand hence to aid 'cu rent c liinec q in'alslietween'R and an betweenfllgan'd RQ 'Terent from saidlcu rrentjsourceimeans,

work results;

cond"si'g' nal foutputterm v 's and wherein-the; reststor'designated R spect to a varying. easurand approximately as I 4 o.( fl 1 )f l where T is the measurand, R5

mentiion ed ju.nction ctively,;the current,in'putjferininal smegma, and V dfR ftjoafj ujnction between R i "ndjc'onnecting-meansjtojde liver a variabl'e'gcur- I t a-ljunct on-be.=

i's'thi valubf ra 'azet 1 reference valueof-the,measurand; kz l-andxfia-re 'con' stants, and the resistor R is selected to satisfy the equation so that the total current 1 is linearly proportional to the measurand.

20. An electrical current transmitting system comprising first means providing a first voltage input signal, second means providing second voltage input signal, a first electrical circuit that includes said first and second means, said first circuit also including feedback resistance means to provide a voltage feedback signal proportional to the total current flow in said first circuit, an amplifier connected to the first circuit and energized by the first circuit for providing an output responsive to voltage unbalance between the first and second input signals and said feedback signal, means connected to the amplifier and responsive to the output of said amplifier for adjusting the current flowing through the first circuit to increase the current through said first circuit, and at the same time increase the current through said feedback resistance means when said first voltage input signal increases with respect to said second voltage input signal and to decrease the current through said first circuit and at the same time decrease the current through said feedback resistance means when said first voltage input signal decreases with respect to said second voltage input signal and to thereby correspondingly increase or decrease the magnitude of the feedback signal as determined by the amount of the current is adjusted, and a voltage reference element means connected to said first circuit for passing a portion of the current flowing through the first circuit to maintain said second voltage which is provided by said second means at a reference value, means to connected said feedback resistance means in said first circuit to provide a change in the magnitude of the feedback signal determined by the amount of current is adjusted to modify the voltage magnitude of the first input signal to tend to reduce any voltage difference between the first and second input signals, said first circuit carrying the total current for the electrical transmitting system, which is representative of the magnitude of said variable condition.

21. The combination of claim 19 and resistance means connected in series with said voltage reference element means and said means for adjusting the current to provide a small positive adjustment signal to the second voltage when the current increases.

22. A measuring system comprising a direct current source, a load, a single pair of wires, said source, load and wires being connected in series so that load current which flows through said load also flows through said pair of wires, current adjustment means for controlling and adjusting the flow of current through said wires and said load, means to connect said current adjustment means in series with said wires, means for providing first and second voltages which differ from each other in accordance with the magnitude of a variable condition, voltage responsive means having an input connected to the last mentioned means to sense differences in said first and second voltages, said voltages responsive means also being connected to said current adjustment means for providing a control signal controlling said current adjustment means to change the current through said wires and said load in accordance with the difference between said first and second voltages, said means to connect including resistance means to provide a feedback voltage that varies as a known function of the value of total current through said load, and means connecting said resistance means to said means for providing the first and second voltages to provide variations in said feedback voltage caused by changes in current through said wires and said load to the input of the voltage responsive means to tend to maintain the difference between said first and second voltages substantially constant so that the changed current flowing through said wires and load as adjusted by said current adjustment means is representative of the magnitude of said variable condition.

23. A current transmitting system having all electrical actuation comprising a circuit having an input terminal and an output terminal, a first DC voltage signal input means for receiving a first DC voltage signal, a second DC voltage signal input means for receiving a second DC voltage signal, amplifier and control means connected in said circuit to be powered solely through said input and output terminals and coupled to both of said input means for controlling the total current flow in said circuit in response to DC voltage signals provided by said first and second voltage signal input means, a DC resistance bridge means coupled to the output of the amplifier and control means and being coupled to said first and second signal input means for providing DC voltage signals to said first and second signal input means, means connected to said bridge to change the voltage at one of the signal input means, a fixed feedback resistor means coupled in said bridge and to said amplifier and control means to be responsive to total current flow between said input and output terminals to provide a voltage signal in the bridge means due to the current flow through said feedback resistor means to reduce any difference between the DC signals at the voltage signal input means as the current between said input and output terminals changes, whereby the changed current flow between said input and output terminals represents the amount of change in the value of the voltage signal provided by said first signal input means with respect to the voltage signal provided by said second signal input means.

" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 859,594 Dated Janna '7 197 Inventor(s) Earl A. arindhflim I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 22 "a jjust" hould be--adjuet--; Column 3, line 23 "Ito" should bo l to--; Column 3, line 44 after inoert assume R Column 3, line 67 "senositive" should be--senoitivo--. Column 4, line 65 before "result-" insert --the--. Column 9, line 13 (Claim 6, line 11) "firth" should be--fifth--; Column 9, line 22 (Claim 7, line 1) "mm" should be--refer--. Column 10, line 35 (Claim 11, line 22) "ones" should he -one. Column 13, line 38 (Claim 20, line 29) "connected" should be--connect--.

Signed and sealed this lst dayof- April 1:75.

fittest:

C. MARSHALL DAISY? l n FUL Commissioner 0?. Patents i-lttestlng flcer and Trademarl'zs 233 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,859,594 Dated Janna 7 1 7 Inventor(s) Earl A. Crindhem It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 22 "adjjust" hould be--adjuat--; Co1umn 3, line 23 "Ito" should bo l to---; Colman 3, line 44 after insert-assume I Column 3 line 67 "sonuitive" should bo--oenoitivo--. Column 4, line 65 before "roault insert "the". Column 9, line 13 (Claim 6, line 11) "firth" should be--fith--;-Co1umn 9, line 22 (Claim 7, line 1) "rotor" should bo--refor--. Column 10, line 35 laim 11, line 22) "onee" should be--one--. Column 13, line 38 (Claim 20, line 29) "connected" should bo -connect".

Signed and sealed this 1st day, or"- .--.pril 1375.

Attest:

C MARSHALL DANE? RUIZ: C, I-ZAE OI'F Commissioner of Patents Attesting diflcer and Tr demark 

1. A measuring system comprising a pair of terminals, a source of direct current and a load, means to connect the source and load in series between said current terminals, means to provide a variable voltage signal which varies in magnitude as a function of a variable condition to be measured, a current adjusting controller means having a voltage responsive input coupled to said variable voltage means for receiving said variable voltage signal and adjusting the current through said terminals as a function of the magnitude of the variable voltage signal at said input, said controller means being connected across said terminals and energized solely from current supplied through said terminals, resistance means connected to said current terminals to provide a voltage feedback which is a function of total current flowing through said terminals, and means connecting said resistance means to said input of said controller means to provide the feedback voltage as determined by the current through said resistance means to tend to balance a change in the variable voltage signal at said input occurring due to a change in magnitude of said variable condition and to stabilize the adjusted current through said terminals at a new magnitude which is representative of a change in magnitude of said variable condition.
 2. The measuring system of claim 1 wherein there is provided resistance network means for providing a variable voltage signal, said network means being connected to said terminals, energized solely through said terminals, and including a first resistor which varies in value as a function of the variable condition to be measured, and comprising at least portions of the first mentioned means to provide a variable voltage signal, the first mentioned resistance means and the means connecting the resistance means to said input.
 3. The measuring system of claim 2 wherein said resistance network means includes additional resistance means, said means to connect said first mentioned resistance means to said input including means to connect said first resistor, said first mentioned resistance means and said additional resistance means in a bridge circuit and to provide bridge voltage output terminals that are connected to said voltage responsive imput.
 4. The combination of claim 2 wherein the first mentioned resistance means comprises a second resistor and the resistance network means additionally comprises third, fourth, and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors.
 5. The combination of claim 4 and a voltage reference means to provide a reference voltage for said network means connected between a junction intermediate said fourth and fifth resistors and a junction intermediate said second and third resistors.
 6. The combination of claim 2 wherein the first mentioned resistance means comprises a second resistor and said resistance network means additionally comprises third, fourth, and fifth resistors, all of said resistor being connected in a closed series loop, said first resistor being connected between the second resistor and fourth resistor and the third resistor connected between the second and fifth resistors, and wherein the voltage reference means is connected for current conduction from the junction of the third and second resistors to the junction of the fourth and firth resistors, the junction of the first and fourth resistors comprising a first voltage terminal and the junction of the fifth and third resistors comprising a second voltage terminal, and the junction of the first and second resistors being coupled to one of said first mentioned pair of terminals and the other of said first mentioned pair of terminals being connected to the junction of said second and third resistors.
 7. The combination of claim 6 and a voltage reeference means connected to said resistance network means and to said first mentioned pair of current terminals to provide a reference voltage for said resistance nEtwork means, including a voltage reference element connected to the junction of said fourth and fifth resistors and a sixth resistor connected between the voltage reference element and the junction of the third and the second resistors and wherein the sixth resistor is connected in series with the second resistor and the first mentioned pair of current terminals.
 8. The combination of claim 7 wherein the voltage reference element is a Zener diode.
 9. The combination of claim 2 wherein said resistance network means includes second, third and fourth resistors connected in a symmetrical bridge arrangement having one end of said first resistor connected to one end of said third resistor and one end of said second resistor being connected to one end of said fourth resistor, the other ends of said first and second resistors being connected to one of said terminals, a fifth resistor having one end connected to the other end of said terminals, and a sixth resistor having one end connected to the junction of said second and fourth resistors and voltage reference means for providing a reference voltage for said resistance network means including a voltage reference element having one end connected to the junction of said third and fourth resistors and the other end connected to the other end of said fifth resistor, and the other end of said voltage reference element being connected to the other end of said sixth resistor.
 10. A transmitter for measuring a variable voltage dependent upon the value of a variable parameter, first and second wires each having a first end portion and a second end portion, a transmitter connected across said second end portions, a measuring device, current source means for providing the sole energy supply to the transmitter and the current that serves to transmit a signal from the transmitter, said measuring device and current source means being connected in series across the first end portions of said wires, said transmitter including means providing a voltage that varies in response to the variation of the parameter to be measured, amplifier means having an input portion coupled to said variable voltage providing means and an output portion for providing an output voltage at said output portion that is dependent on the magnitude of the variable voltage at said input portion, means coupled to the output portion and the second end portions of said wires for regulating the flow of current through the wires as a function of the output voltage including feedback resistor means coupled across the second end portions of the wires for generating a feedack voltage dependent upon the current flowing in said wires, and means for electrically connecting said feedback resistor means to the input portion of said amplifier means to tend to balance variations in magnitude of the variable voltage at said input portion.
 11. The combination as specified in claim 10 wherein said means providing a voltage that varies in response to variation of the parameter to be measured comprises a resistance network including a first resistor which varies in resistance as a function of a parameter to be measured, said feedback resistor means comprising a second resistor in said resistance network, said resistance network further including third and fourth resistors, said first, second, third, and fourth resistors being connected in a symmetrical bridge arrangement having one end of said first resistor connected to one end of said third resistor to provide a first voltage terminal, and one end of said second resistor being connected to one end of said fourth resistor to provide a second voltage terminal, and wherein the other ends of said first and second resistors are connected to one of said wires and the other ends of said third and fourth resistors are connected together to the other of said wires, said feedback resistor means further including fifth and sixth resistors and a voltage reference means for providing a reference voltage for said resistance network including A voltage reference element having one end connected to the junction of said third and fourth resistors and the other end connected through said fifth resistor to the other of said wires and through said sixth resistor to said second voltage terminal.
 12. An electrical current transmitting system comprising a pair of current terminals adapted for connection to a load and power source that are in series; a resistance network means connected in series with said current terminals to be powered and excited solely therethrough for providing a first variable voltage signal which varies as a function of the magnitude of a physical condition to be measured; a feedback resistance means connected to said resistance network means for providing a second voltage signal which is a function of the current through said feedback resistance means; means to be powered solely through said terminals connected to the feedback resistance means and to the current terminals to establish a current flow path between said terminals and to provide a known portion of current flowing between said terminals through said feedback resistance means including an amplifier means having a voltage input portion and an output portion providing an output signal which is a function of the voltage at said voltage input portion, and current control and adjustment means coupled to said output portion to receive the output signal and to adjust the current in said current flow path between said terminals as a function of said output signal; and means to couple said resistance network means to said voltage input portion to provide a voltage at said input portion of said amplifier means which is a function of said first and second voltage signals.
 13. The combination of claim 12 further characterized in that a voltage reference means is connected to said resistance network means in series with said feedback resistance means between said terminals to provide a reference voltage for said resistance network means.
 14. The combination of claim 13 wherein said network means includes a first resistor, and said voltage reference means includes a fixed resistance and a semiconductor reference element in series with said fixed resistance, the series combination of said reference element and fixed resistance being connected across said first resistor so that an increase in excitation voltage across said first resistor is provided as a result of an increased current through said voltage reference means.
 15. The combination of claim 12 wherein said amplifier means comprises a chopper driver having a driver output terminal; said means to couple comprising a modulator means having first and second input terminals connected to said resistance network means to sense voltages in said network means, a third input terminal capacitively connected to the drive output terminal and a modulator output terminal; an operational amplifier having an input terminal connected to the modulator output terminal and having an output terminal; a demodulator having a first input terminal connected to the amplifier output terminal, a second input terminal capacitively connected to the driver output terminal and having an output terminal; and said current control and adjustment means comprising a current control stage having a first input terminal connected to the demodulator output terminal, a second current terminal connected to one of the transmitting system pair of current terminals and a third terminal means connected to pass current through portions of said resistance network including said feedback resistance means to the other of the transmitting system pair of current terminals.
 16. A signal transmitter comprising a pair of wires, a load, current supply means for providing a load current through said load and wires connected in series with the load across first ends of said wires, a current transmitter connected across said wires at the other end thereof, the current flowing through said wires serving as the sole power supply to The transmitter as well as serving at the load to represent the signal transmitted from said transmitter, said transmitter including an input circuit means connected to said two wires to provide a variable voltage which tends to change in response to changes in a variable condition constituting the information to be transmitted and voltage responsive current controller means connected to the two wires and the input circuit means for adjusting the current through said two wires in response to the voltage supplied by the input circuit means, feedback resistor means connected to said current controller means and in series with said wires to have at least a known portion of load current carried by said wires flowing therethrough to provide a feedback voltage dependent on the magnitude of said load current, and means connecting said feedback resistor means to the input circuit means to provide a feedback voltage in opposition to the variable voltage so that the input voltage to the current controller means is dependent upon the variable voltage and the feedback voltage whereby adjusted load current flowing through said wires and load is representative of the magnitude of said variable condition.
 17. The transmitter of claim 16 wherein the current controller means comprises a chopper driver having a driver output, a modulator having input terminals connected to the input circuit means, means coupling the driver output to said modulator, an amplifier having an input and an output and having its input connected to the modulator output, a demodulator having an input connected to the amplifier output, means coupling the driver output to the demodulator, and a current control stage having a first input terminal connected to the demodulator output, a second input terminal connected to one of said wires and an output terminal connected to the other of said wires.
 18. The signal transmitting system of claim 16 wherein said input circuit means comprises a resistance network means to provide a variable voltage including a first resistor which changes in value in response to a change in a variable condition and said feedback resistor means comprises a second resistor also coupled to said resistor network means, said resistance network means further including third, fourth and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors, and a voltage reference element and a sixth resistor connected in series between the junction of said fourth and fifth resistors and the junction of said second and third resistors, said means to couple said input circuit to said two wires comprising means coupling one of said wires to the junction of the fourth and fifth resistors to carry a first portion of the current to said resistance network and means also coupling the junction of the voltage reference element and the sixth resistor to said one wire to carry a second portion of the current in the one wire to the resistance network, said last-mentioned junction being connected through portions of said resistance network to the other wire and hence to said current supply means.
 19. An electrical network comprising six resistors and a source of constant voltage connected together and including first and second signal output terminals, a current input terminal and a current output terminal and means providing a source of current to said current input terminal, wherein the six resistors designated R1 through R6 and the voltage source designated VC are connected to be equivalent to an arrangement having R1, R2, R3, R4, R5 connected in a closed series loop in the order cited having the first and second signal output terminals between R1 and R4 and between R2 and R3 respectively, the current input terminal between R1 and R2, and the current output terminal between R4 and R5, and the source VC serially connected with R6 from a junction between R1 and R2 to a junction between R3 and R5, and connecting means to deliver a variable current from said current source means to a junction between VC and R6 so that the total current designated IT delivered to the network which results in zero voltage difference at the first and second signal output terminals is IT VC( R2 (R4 + R5) - R1R3)/( R1( R5 (R2 + R3) + R6 (R3 + R5))- R2R4R6) and wherein the resistor designated R4 varies with respect to a varying measurand approximately as R4 R0 (1 + Alpha T - Beta T2) where T is the measurand, R0 is the value of R4 at a zero reference value of the measurand, Alpha and Beta are constants, and the resistor R6 is selected to satisfy the equation
 20. An electrical current transmitting system comprising first means providing a first voltage input signal, second means providing a second voltage input signal, a first electrical circuit that includes said first and second means, said first circuit also including feedback resistance means to provide a voltage feedback signal proportional to the total current flow in said first circuit, an amplifier connected to the first circuit and energized by the first circuit for providing an output responsive to voltage unbalance between the first and second input signals and said feedback signal, means connected to the amplifier and responsive to the output of said amplifier for adjusting the current flowing through the first circuit to increase the current through said first circuit, and at the same time increase the current through said feedback resistance means when said first voltage input signal increases with respect to said second voltage input signal and to decrease the current through said first circuit and at the same time decrease the current through said feedback resistance means when said first voltage input signal decreases with respect to said second voltage input signal and to thereby correspondingly increase or decrease the magnitude of the feedback signal as determined by the amount the current is adjusted, and a voltage reference element means connected to said first circuit for passing a portion of the current flowing through the first circuit to maintain said second voltage which is provided by said second means at a reference value, means to connect said feedback resistance means in said first circuit to provide a change in the magnitude of the feedback signal determined by the amount the current is adjusted to modify the voltage magnitude of the first input signal to tend to reduce any voltage difference between the first and second input signals, said first circuit carrying the total current for the electrical transmitting system, which is representative of the magnitude of said variable condition.
 21. The combination of claim 19 and resistance means connected in series with said voltage reference element means and said means for adjusting the current to provide a small positive adjustment signal to the second voltage when the current increases.
 22. A measuring system comprising a direct current source, a load, a single pair of wires, said source, load and wires being connected in series so that load current which flows through said load also flows through said pair of wires, current adjustment means for controlling and adjusting the flow of current through said wires and said load, means to connect said current adjustment means in series with said wires, means for provIding first and second voltages which differ from each other in accordance with the magnitude of a variable condition, voltage responsive means having an input connected to the last mentioned means to sense differences in said first and second voltages, said voltage responsive means also being connected to said current adjustment means for providing a control signal controlling said current adjustment means to change the current through said wires and said load in accordance with the difference between said first and second voltages, said means to connect including resistance means to provide a feedback voltage that varies as a known function of the value of total current through said load, and means connecting said resistance means to said means for providing the first and second voltages to provide variations in said feedback voltage caused by changes in current through said wires and said load to the input of the voltage responsive means to tend to maintain the difference between said first and second voltages substantially constant so that the changed current flowing through said wires and load as adjusted by said current adjustment means is representative of the magnitude of said variable condition.
 23. A current transmitting system having all electrical actuation comprising a circuit having an input terminal and an output terminal, a first DC voltage signal input means for receiving a first DC voltage signal, a second DC voltage signal input means for receiving a second DC voltage signal, amplifier and control means connected in said circuit to be powered solely through said input and output terminals and coupled to both of said input means for controlling the total current flow in said circuit in response to DC voltage signals provided by said first and second voltage signal input means, a DC resistance bridge means coupled to the output of the amplifier and control means and being coupled to said first and second signal input means for providing DC voltage signals to said first and second signal input means, means connected to said bridge to change the voltage at one of the signal input means, a fixed feedback resistor means coupled in said bridge and to said amplifier and control means to be responsive to total current flow between said input and output terminals to provide a voltage signal in the bridge means due to the current flow through said feedback resistor means to reduce any difference between the DC signals at the voltage signal input means as the current between said input and output terminals changes, whereby the changed current flow between said input and output terminals represents the amount of change in the value of the voltage signal provided by said first signal input means with respect to the voltage signal provided by said second signal input means. 